1. Field of the Invention
The present invention relates to an eyepiece lens and a viewfinder optical system.
2. Description of the Prior Art
It is known to use a diffractive optical surface in an optical system. For example, U.S. Pat. No. 5,148,314 proposes an optical system consisting of a single lens having a diffractive optical surface, and so does a treatise by G. J. Swanson and W. B. Veldkamp, titled "Infrared Applications of Diffractive Optical Elements" SPIE proceedings, vol. 885, paper 22 (1988). On the other hand, U.S. Pat. Nos. 5,044,706 and No. 5,446,588 propose viewfinder optical systems that consist of an objective optical system and an eyepiece optical system and that correct chromatic aberration by the use of a diffractive optical surface. U.S. Pat. No. 5,044,706 proposes a viewfinder optical system in which the objective optical system has a diffractive optical surface, and U.S. Pat. No. 5,446,588 proposes a viewfinder optical system in which the eyepiece optical system has a diffractive optical surface.
In an example described in the above-mentioned treatise by Swanson et al., a single lens having a diffractive optical surface is used to correct chromatic aberration. In this example, the lens acts as an aperture diaphragm, and accordingly the position of the aperture diaphragm coincides with the position of the lens. In a practical viewfinder optical system, however, if the eyepiece lens is composed of a single lens, the pupil (which corresponds to the aperture diaphragm) is located away from the lens, and therefore chromatic aberration (in particular, lateral chromatic aberration) appears with a different pattern than in the example of Swanson et al.
On the other hand, in the viewfinder optical systems proposed in the above-mentioned U.S. Pat. No. 5,044,706 and U.S. Pat. No. 5,446,588, the eyepiece lens is composed of a plurality of lenses to achieve proper correction of aberration. By contrast, when the eyepiece lens of a viewfinder optical system is composed of a single lens, aberration needs to be corrected with only two lens surfaces, and therefore it is necessary, not only to use a diffractive optical surface to achieve proper correction of chromatic aberration, but also to elaborately design the power of the diffractive optical surface and the shapes of the lens surfaces to achieve proper correction of spherical aberration, coma aberration, astigmatism, curvature of field, and distortion.
From the viewpoint of the efficiency of diffraction of a diffractive optical surface, as well as the flare caused by rays resulting from diffraction of orders other than designed, it is desirable to use as few diffractive optical surfaces as possible. In the optical system proposed in the above-mentioned U.S. Pat. No. 5,148,314, the power of a diffractive optical surface and its distance from the aperture diaphragm are optimized (i.e. exactly defined) so that chromatic aberration (i.e. longitudinal chromatic aberration and lateral chromatic aberration) will be minimized with as few diffractive optical surfaces as possible. However, in an eyepiece lens, the pupil acts as an aperture diaphragm, and therefore the position of the aperture diaphragm (i.e. the position of the pupil) is not fixed. Accordingly, this optical system, when used as an eyepiece lens, does not achieve proper correction of chromatic aberration, as long as it is composed of a single lens.
In either of the optical systems proposed by the above-mentioned U.S. Pat. No. 5,044,706 and U. S. Pat. No. 5,446,588, the lens having the diffractive optical surface is given a strong power relative to the power of the entire viewfinder optical system, that is, the diffractive optical surface is used simply to correct chromatic aberration. The inventors of the present invention, however, noticed that the power of a lens having a diffractive optical surface can as well be used to solve the following problems associated with a viewfinder optical system as described above.
In general, when a viewfinder optical system is incorporated into a camera, the viewfinder optical system is, on its front and rear sides, additionally fitted with outer covering members such as protection glasses that are used as eyepiece-side and objective-side windows of the viewfinder of the camera. If parts of the viewfinder optical system can be directly fitted to the outer covering of the camera, there will be no need to use separate outer covering members such as the above-noted protection glasses, and, as a result, it is possible to reduce the cost of the camera. However, fitting optical members such as a viewfinder optical system directly to structural members such as outer covering members is not practical for the following reasons.
Optical members are far more sensitive to fitting errors than structural members, that is, optical members require far higher fitting accuracy than structural members. Accordingly, fitting an optical member directly to an outer covering member or any other elastic member of the camera results in poorer optical performance than designed.
Moreover, as the power of an optical member becomes stronger, it becomes more sensitive to fitting errors, that is, it requires higher fitting accuracy. In the optical systems of the patents cited previously, the lens having the diffractive optical surface is given a strong power, and accordingly it requires considerably high fitting accuracy. This means that, even though the diffractive optical surface serves to correct chromatic aberration properly, its use is meaningless as long as the lens is fitted to a more roughly fitted member of the due to a considerable degradation of optical performance.
If it were possible to fit a lens having a diffractive optical surface to a more roughly fitted member without degrading optical performance, then it would be possible to achieve not only proper correction of chromatic aberration, but also reduction of the number of the components, and thus the cost, of the camera. The possible advantages of this constitution are such that the inventors of the present invention were motivated to study it more closely until led to the conclusion that it is in fact possible to fit a lens having a diffractive optical surface to a more roughly fitted member if the power of that lens is made weaker than in conventional viewfinder optical systems but not weaker than is necessary to achieve sufficient correction of chromatic aberration.
A Kepler-type real-image viewfinder optical system in which the image of an object is formed by an objective lens and is then magnified by an eyepiece lens for observation is provided with an erecting optical system such as an inverting prism to reflect the vertical and horizontal directions of the object correctly in the observed image. The use of the erecting optical system, however, inevitably makes the viewfinder optical system as a whole larger.
It is known that a lens system can be made compact by providing a curved surface on a prism. For example, U.S. Pat. No. 5,206,675 proposes an example in which a Porro prism is provided with a curved surface to omit a separate eyepiece lens.
However, providing a prism with a curved surface causes large chromatic aberration.